During semiconductor device fabrication, several polishing processes can be used to prepare a substrate surface for the next integrated structure. The polishing process typically used is one of mechanical polishing (“MP”), chemical polishing also referred to as etchback, and chemical-mechanical polishing (“CMP”). Collectively, these polishing operations can be referred to as CMP. CMP is frequently used as it combines chemical action with mechanical shear and sweep action to remove material from the upper surface of a wafer.
The CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other features, many substrates develop large step heights that create highly topographic surfaces across the substrates. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing the microelectronic devices.
It is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate as quickly as possible. The throughput of CMP processing is a function, at least in part, of the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate is planar and/or when enough material has been removed from the substrate to form discrete components on the substrate (e.g., shallow trench isolation areas, contacts, damascene lines, etc.). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is under-planarized, or components on the substrate may be destroyed if it is over-polished. Thus, it is highly desirable to stop CMP processing at the desired endpoint.
In one conventional method for determining the endpoint of CMP processing, the planarizing period of a particular substrate is estimated using an estimated polishing rate based upon the polishing rate of identical substrates that were planarized under the same conditions. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate and other variables may change from one substrate to another. Thus, this method may not produce accurate results.
Other methods for determining the endpoint of CMP processing include using an on-line Kalman filter have been demonstrated. For example, Vincent et al. in J. Electrochem. Soc. Vol. 144, No. 7, pp. 2467-2472 (July 1997), Vincent et al. in Mat. Res. Soc. Symp. Proc. Vol 406, pp. 87-93 1996, Vincent et al. in IEEE Transactions on Semiconductor Manufacturing, Vol. 10, No. 1, pp. 42-51 (February 1997), and Vincent et al. in ISSN1083-1312/971/1701-04274 (1997), herein incorporated by reference, teach on-line use of a Kalman filter to promote a preferred end-point during CMP on a wafer. However, they do not teach deconvolution of array and periphery data on a patterned wafer.
In another method for determining the endpoint of CMP processing, the substrate is removed from the pad and then a measuring device measures a change in thickness of the substrate. Removing the substrate from the pad, however, interrupts the planarizing process and may damage the substrate. Thus, this method generally reduces the throughput of CMP processing.
Other teachings of interest include Yueh (U.S. Pat. No. 5,865,665) and Hoffman (U.S. Pat. No. 6,290,572), and are also incorporated herein by reference. In one method, control of a process parameter of a planarizing cycle is done by predicting a thickness of an outer film over a first region on the substrate assembly and providing an estimate of an erosion rate relationship based on a first erosion rate over the first region and a second erosion rate over a second region. The erosion rate relationship can be the first and second erosion rates or an erosion rate ratio between the first and second erosion rates. The first region can be an array at a first elevation and the second region can be a periphery area at a second elevation.
The endpointing procedure continues by determining an estimated value of an output factor, such as a reflectance intensity from the substrate assembly. The output factor can be estimated by modeling the output factor based upon the thickness of the outer layer over the first region and the erosion rate ratio between the first region and the second region. The endpointing procedure continues by ascertaining an updated predicted thickness of the outer film over the first region by measuring an actual value of the output factor during the planarizing cycle without interrupting removal of material from the substrate, and then updating the predicted thickness of the outer film according to the variance between the actual value of the output factor and the estimated value of the output factor. The endpointing process also continues by repeating the determining procedure and the ascertaining procedure using the revised predicted thickness of the outer layer of an immediately previous iteration to bring the estimated value of the output factor to within a desired range of the actual value of the output factor. The planarizing process is terminated when the updated predicted thickness of the outer layer over the first region is within a desired range of an endpoint elevation in a substrate assembly.
What is needed in the art is a method of operating a physical process with results that overcome at least one of the challenges of the prior art.